Fire Weather
Current Fire Danger Level: 2 Moderate
When the fire danger is "moderate" it means that fires can start from most accidental causes, but the number of fire starts is usually pretty low. If a fire does start in an open, dry grassland, it will burn and spread quickly on windy days. Most wood fires will spread slowly to moderately. Average fire intensity will be moderate except in heavy concentrations of fuel, which may burn hot. Fires are still not likely to become serious and are often easy to control.
Local Thresholds
A fire weather watch or Red Flag warning may be issued when some or all of the below criteria are met:
20’ Winds Exceeding 8 MPH
Relative Humidity (RH) below 10%
Temperatures exceeding 89°F
A Burning Index (BI) exceeding 62
Preparedness Level (PL)
The preparedness level, or staffing level (SL) for a given area represents the number of commitments the area has on emergency incidents. Preparedness level ranges from 1 to 5, with 1 being the lowest and 5 being the highest. This metric is often used to show the relative percentage of units currently committed to emergency incidents. Click here for a detailed breakdown of the National Preparedness Levels.
Fire Danger Variable Definitions
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The Energy Release Component, ERC, is a metric that measures the energy per area (BTU / Square ft.) released by the flame front at the head of a fire. The ERC considers the previous seven days of fire weather and fuel moisture in its calculation. For more information, see this ERC Fact Sheet by the US Forest Service.
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Ignition Component is related to the probability of an ignition source producing a fire that will require suppression action or is self sustaining. It is mainly a function of the 1-hour time lag (fine fuels) fuel moisture content and the temperature of the receptive fine fuels. Ignition Component is expressed as a probability from 0% to 100%.
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Spread Component is a rating of the forward rate of spread at the head of a fire. Spread Component integrates the effect of wind, slope, and fuel properties. The daily variations are caused by the changes in the wind and moisture contents of the live fuels and the 1-Hour, 10-Hour, and 100-Hour dead fuel time lag classes. Spread Component is roughly equivalent to theoretical fire spread rate in feet per minute.
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Burning Index is a measure of fire intensity. Burning Index combines the Spread Component (SC) and Energy Release Component (ERC) to relate to the contribution of fire behavior to the effort of containing or supressing a fire. Burning Index has no units, but in general, it is roughly equivalent to 10 times the flame height of a fire.
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The Hot Dry Windy Index (HDW) is a multiplication of the maximum wind speed and maximum vapor pressure deficit (VPD) in the lowest 50 or so millibars in the atmosphere. Because HDW is affected by heat, moisture, and wind, seasonal and regional variability is common. HDW is good for measuring the general likeliness of adverse atmospheric conditions that may make fire suppression more difficult. More information can be found on the Hot Dry Windy Index website.
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Fuel Moisture is broken up into four different categories. Each of these categories are called time lag classes. Each time lag class represents different fuel types and the general amount of time that it takes for 2/3 of the fuel to respond to changes in atmospheric moisture. For instance, fuels in the 10-hour time lag class will take roughly 10 hours to adjust to a more moist or more dry atmospheric condition. More information on fuel moisture, as well as time lag classification information, can be found on the National Centers for Environmental Information Website.
Additional Resources